With the remarkable development of small, thin, and high-functionality portable electronic equipment such as mobile phones and PDAs, there is a growing demand for smaller, thinner, and high-capacity batteries as their power source. Lithium ion rechargeable batteries can be designed small and high-capacity, and in particular, flat prismatic types are suitable in making the equipment thinner; they have therefore been increasingly used as the repeatedly usable rechargeable battery for portable electronic equipment.
Because lithium ion rechargeable batteries have high energy density and contain a flammable organic solvent as electrolyte, it is essential to take account of safety measures. They must have such safety features as to ensure that no damage is caused to the equipment or injury to the user in the event that an abnormality arises for some reason. For example, if the positive and negative terminals of the battery are short-circuited for some reason, a large short-circuit current flows in high energy density batteries, whereupon the inner resistance generates Joule heat and the battery temperature rises. A temperature rise in the battery leads to a rapid increasing of inner gas pressure caused by reactions between positive electrode active materials and electrolyte, or from evaporation or decomposition of electrolyte, which results in fire or explosion of the battery. Batteries may fall into a high-temperature state not only because of external short-circuiting but also of overcharge; the same applies if the portable electronic equipment loaded with the battery is placed near a heater or left inside a car parked in a hot weather environment.
A battery abnormality can be induced by any of electrical, mechanical, or thermal factors; thus non-aqueous electrolyte batteries represented by lithium ion rechargeable batteries are provided with safety features for preventing batteries from falling into an abnormal state and for evading a further dangerous state even if an abnormality should arise. Such features are usually incorporated in batteries as their own natures; for example, active materials on the electrodes and electrolyte may be made not to be excessively reactive, or, a polyolefin porous film may be employed for the separator because of its “shutdown function,” in which minute pores are softened and close under an abnormally high temperature. Cylindrical lithium ion rechargeable batteries are usually provided with a protective feature such as a Positive Temperature Coefficient (PTC) element connected in series to the input/output circuit at the sealing end, which limits current flow in the event of external short-circuiting. Batteries that do not have a sufficient space for the PTC element inside are normally provided with a PTC element or temperature fuse as outside circuit components. Further, a circuit for protecting the battery from overcharge and over discharge is an absolute requirement. In general, these constituent elements are all packed with the battery inside a pack case to form a battery pack.
However, battery packs using pack cases are not suited to portable electronic equipment that are re-modeled in short cycles, because the manufacturing cost of molding dies used in the resin molding of pack cases tends to be high, and the time required for designing new molding dies is relatively long. Battery packs with resin-molded outer cases also have limitations in making portable electronic equipment smaller and thinner because of the limitations on the moldable thickness in the resin molding process.
Furthermore, in order to prevent the user from disassembling a battery pack for wrong use or for satisfying curiosity, it must have a design that is hardly disassemblable, or a design that alerts the user that it has been disassembled. Taking account that the battery packs are used for portable electronic equipment, they also need to have a rigid structure that can withstand vibration or shocks in a falling accident, and a moisture resistance, particularly for the electronic circuit parts. In achieving the structure having a disassemblability, a certain rigidity, and a moisture resistant, the idea has emerged that a battery may be united with a circuit substrate including a battery protective circuit by resin molding.
Such resin-molded battery packs described above are disclosed in Japanese Laid-Open Patent Publications Nos. 2002-134077 and 2002-166447, in which a battery and a circuit substrate are connected by a connecting member to form an intermediate product, which is placed inside a die, and resin is filled around the intermediate product such as to expose external terminals formed on the circuit substrate to the outside.
Japanese Laid-Open Patent Publication No. 2000-315483 discloses a structure in which a battery and a circuit substrate are connected by a connecting member and placed inside a die, and the circuit substrate is resin-sealed and fixed on the battery or its pack case (battery lid), or both the circuit substrate and the battery are resin-sealed.
Battery packs of lithium ion rechargeable batteries are normally provided with a battery protection feature that prevents a temperature rise caused by external short-circuiting or overcharge as mentioned above, and in addition, they are provided with a heat sensitive element such as a temperature fuse or PTC element that cuts the battery circuit as a backup safety feature in the event that the protective feature has not functioned.
Because the heat sensitive element is heat-coupled to the rechargeable battery so that it operates not only in an over current condition but also upon a change in the battery temperature, and connected to the circuit that connects the rechargeable battery with the circuit substrate, a measure must be taken so that the heat sensitive element is not destroyed by the heat of the resin filled between the rechargeable battery and the circuit substrate during the resin molding. Generally, temperature fuses have a fusion temperature of 104° C.; on the other hand, the temperature of molten resin, even though it is a hot melt resin that melts at a relatively low temperature, exceeds 200° C. While hot melt resins have a lower melting temperature than other molding resins and allow easy handling, the melting temperature is still much higher than the fusion temperature of the temperature fuse. A direct contact of molten hot melt resin with the temperature fuse will certainly induce fusion, whereby the circuit in the battery pack will be cut and not properly function. In the case with a PTC element, hot resin of more than 200° C. will change the bridging structure of the conductive polymer that is a chief component of the PTC element, whereby the temperature-current characteristics and trip temperature will be affected and the reliability of the PTC element will be deteriorated. Accordingly, in the production of battery packs of a rechargeable battery and a circuit substrate united by filling with resin with safety features using heat sensitive elements, it is an absolute requirement to provide measures to prevent destruction of the heat sensitive elements.
It is an object of the invention to provide a battery pack of a battery and a circuit substrate that are united by resin molding, the battery pack having a structure that prevents a heat sensitive element provided as a safety feature from being destroyed by filled resin, and a manufacturing method thereof.